1. Field of the Invention
The present invention relates to catheter devices for continuous analysis of body fluids and, in particular, relates to a long indwelling double bore catheter which may be inserted into a fluid carrying member of a living being for long periods of time to provide a continuous fluid flow for analysis.
2. Description of the Prior Art
In hospital and clinical establishments, as well as in research facilities, it is frequently necessary to slowly withdraw blood samples from a human patient or an experimental animal over an extended period of time, analyze the blood and, in some cases, return the blood to the patient or animal. This is conveniently accomplished by inserting a flexible tube cannula into a blood vessel, such as a vein or an artery, of the patient or animal and withdrawing blood as desired from the cannula. In order to prevent blood coagulation in the cannula or in associated tubing for conducting the blood sample from the cannula, it is usual practice to mix an anticoagulant material, such as heparin, with blood sample as soon as possible after it is withdrawn from the blood vessel. For this purpose, a double lumen cannula apparatus has been used for many years. In this double lumen apparatus a small diameter tube is inserted within the usual cannula sheath. The passage through this small diameter tube forms one lumen, and the annular space around the outer wall surface of the inner tube and the inner wall surface of the cannula sheath forms the other lumen. The inner tube terminates within the cannula sheath a short distance from the tip of the cannula sheath. In use, the anticoagulant material passes through the annular lumen toward the tip of the cannula sheath where it contacts and mixes with the blood entering the cannula from the blood vessel. The mixture of blood and anticoagulant material then flows out of the cannula through the inner tubular lumen of the double lumen cannula apparatus.
The prior art double lumen cannula apparatus typically has two principal problems. First, the distance between the end of the inner tubular lumen and the tip of the cannular sheath is not controlled with any high degree of precision. Most double lumen cannula apparatuses have a support body through which the inner tubular lumen and a conduit for anticoagulant material are passed. The cannula sheath, which normally has a funnel portion at one end and a tubular portion at the other end, is mated in conical press fit against the body so that the inner tubular lumen is coaxially within the tubular portion of the cannula sheath. Any variations in the length of the cannula sheath and/or of the inner tubular lumen or of the coaxial press fit between the cannular sheath and body can cause a variation in the distance between the end of the inner lumen and the tip of the cannula sheath. If this distance is undersirably short, some of the anticoagulant material from the annular lumen undesirably enters the blood vessel of the patient or animal. There could also be inadequate dilution of the blood sample by the anticoagulant material. This would cause false readings when the diluted blood sample is analyzed. If the distance is undesirably long, some of the blood may start to coagulate in the cannula sheath before it contacts the anticoagulant material.
When the cannula sheath mates against a support body, any variation in the final position of the mated parts from the desired position can cause an undesirable distance variation. Since the cannula sheath generally mates in a conical press fit over an extension of the main support body in most prior art apparatus, there is often considerable variation in the final position. This is caused both by tolerance in the tapers of the sheath and body and also by variations in the manual pressure employed to mate the sheath against the body. Prior art apparatus often employs a resilient material for the support body and its extension. The resiliency of this structural element can also cause undesirable dimensional variations in the assembled apparatus.
A second disadvantage of prior art apparatus is in the excessive volume of the introduction chamber for the anticoagulant material. In typical double lumen cannula apparatus the space in the funnel portion of the cannula sheath between the support body and the junction between the funnel and the tubular portion of the cannula sheath forms an introduction chamber for such material. In normal use, the cannula sheath is first inserted into the blood vessel. Blood begins to flow through the tubular portion and into the funnnel section of the cannular sheath. The catheter portion consisting of a support body, a first conduit forming an inner tubular lumen and a second conduit to provide the anticoagulant material is then inserted into the cannula sheath with the cannula sheath mating against the support body. The volume of blood initially in the cannula sheath and especially in the funnel portion occupies a space intended for introduction of the anticoagulant material and should be minimized to prevent slow response to changes in concentration of an analyte in the blood, as would occur with a large mixing chamber or other on line "dead volume" space. In addition, this volume has considerable variation in prior art apparatus. Moreover, movement of an inner lumen relative to an outer lumen can change flow rates in many prior art devices.
With the development of the double lumen cannula apparatus for blood sampling as set forth in U.S. Pat. No. 4,069,814, improved control over the dilution of a blood sample with anticoagulant material was achieved. Nevertheless, this catheter cannot be used effectively in situations requiring access to large, deep vessels, particularly in cases where small surface veins only are available. In these cases, it is necessary to use a longer catheter such that a deep vein can be accessed. Moreover, in some situations it is necessary to leave the catheter in for a long period of time and in those situations it is also advantageous to use a larger, deeper vein.
If one utilizes double bore tubing to achieve a long indwelling catheter, the diluent being pumped through one bore and the blood being pumped through the other bore, it is found that the diluent spills into the blood stream and reliably diluted blood samples cannot be obtained. If one restricts the opening beyond the tip the situation is improved, but the resulting device is still unsuitable for reliable use. Pulsations caused by the pumping means used will still cause periodic loss of diluent through the end opening.
There is also a need for a double lumen catheter which can be used as effectively in an upright position as in an inverted position, i.e., a catheter which is not position sensitive and, therefore, which does not change its flow rate with each change in the position of the patient.